Method of producing radio wave reflector cords of varied length



C. A. UCI

April 7, 1959 GREGORY NG RADIO WAVE R 2,881,425 METHOD OF PROD EFLECTOR'CORDS OF VARIED LENGTH Filed March 19. 1954 FIG. I

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GREGORY ATTORNEYS METHOD OF PRODUCING RADIO WAVE RE- FLECTOR CORDS OFVARIED LENGTH Charles A. Gregory, Arlington, Va., assignor to the UnitedStates of America as represented by the Secretary of the Navy of Americafor governmental purposes without the payment of any royalties thereonor therefor.

This invention relates to a system and the production of means for amore effective airborne electronic countermeasure for military purposesby creating a false echo from enemy radar waves.

Window or chaff is the code name applied to airborne electronicreflector materials used for military purposes as a countermeasureagainst enemy radar. Generally, window, which comprises strips ofaluminum of various lengths, is dropped from an aircraft to provide areflector for enemy radar waves which will create a false echo, i.e., anecho which is not from the target.

Window, as presently produced, is cut from thin rolled sheet aluminum inthin strips of lengths to correspond to a deslre radio or radar wavelength against which it is used; or is cut in long ribbons approximatelyonehalf inch wide and five hundred feet long, and dispensed from rollsfor use against long wave length radio or radar waves. The disadvantagesof the current methods of production of window, and of the materialsused, are that even though made of a light weight electrical conductingmaterial, the weight and space of the materials become limiting factorswhen large quantities, as required for military use, must be carried anddispensed from aircraft. Furthermore, the solid aluminum ribbon-likematerial does not permit its preparation in electrical lengths tocorrespond to the longer wave length radar frequencies without breakingthe continuity of the roll. Another disadvantage of current and priorart window is that in the event of war, aluminum becomes a criticalmaterial at the time when millions of tons of window is required.

It is an object of this invention, therefore to provide resonant dipolereflectors in short lengths of much smaller cross section than presentlyemployed, and to produce long ribbon material that can be electricallybroken into resonant lengths, both types of which will create muchgreater echo power for the same weight per volume than the materialspresently used, and to effect a saving in critical material (aluminum)in time of war.

Another object of this invention is the provision of radio or radar wavereflector having a base of light weight non-conducting material andhaving aluminum deposited thereon in desired electrical lengths.

Still another object of this invention is to provide a simple andinexpensive method of packaging window which will provide a saving ofspace when carried in military aircraft.

A still further object of this invention is the provision of an airbornecountermeasure radio or radar wave reflecting material which may bepackaged in a small volume.

Still other objects and advantages of this invention will becomeapparent to those skilled in the art after a consideration of thefollowing detailed description and the Patented Apr. 7, 1959 appendedclaim taken in connection with the accompanying drawings in which:

Fig. l is a plan view of this invention;

Fig. 2 is a side elevation of the embodiment shown in Fig. 1;

Fig. 3 is a plan view of a second preferred embodiment of thisinvention;

Fig. 4 is an end in Fig. 3;

Fig. 5 is a View illustrating one method of the product of thisinvention; and

Figs. 6 and 7 are views illustrating a second method of packaging theproduct of this invention.

Referring to Figs. 1 and 2, it is seen that in a first embodiment ofthis invention, a very thin metallic coating 10 of an electricallyconductive material is applied on a non-conducting ribbon base 11.Although aluminum is preferred because of its light weight and highreflecting qualities, metals such as copper and zinc may be readilyemployed. The base 11 is preferably a non-conducting substrate materialsuch as tissue, cellulose, nylon, or other light weight non-conductingmaterial. The metallic coating 10 may be applied to the base 11 by thevacuum evaporation process or by any suitable method. It is also seenthat the metallic coating 10 is broken by the spaces 12 intopredetermined electrical lengths. This may be accomplished by maskingduring the coating process, or by removing portions of the metalliccoating 10, after the material has been applied to the base.

By breaking the metallic coating 10 into lengths corresponding to adipole, or the half wave length of a given radio or radar wave, aresonant reflector which is ideally suited for lower frequency waves isproduced. For example, a length of coating approximately 56 inches longwill resonate at radio frequencies of 100 megacycles. dispensing ofsolid aluminum strips proved unsatisfactory since the individual stripsintertwine and the resonating qualities are, therefore, lost. Thisinvention makes it possible to dispense the individual strip in one longribbon like streamer from a roll. Although the greatest advantages ofthis embodiment are demonstrated in the lower frequency ranges, itappears obvious that the metallic coating may be made of any length toresonate at any given radio or radar frequency.

The embodiment shown in Figs. 3 and 4, to which reference is now made,consists of a base 14 composed of materials similar to that employed inthe base 11 of the embodiment of Figs. 1 and 2, but made in the shape ofa fibrous cord. The metallic coating 15 is applied by the vacuumevaporation process, or any other suitable method as in Fig. l andmaterials such as aluminum, copper or zinc may similarly be used. Unlikethe embodiment of Figs. 1 and 2, the metallic coating in this form ofthe invention is not broken. The coated fibers are cut into lengthscorresponding to a dipole, or one-half wave length, :of the radio orradar wave. This type of reflector is particularly useful in frequencyInegacycles per second since a dipole at is only approximately of aninch long.

The reason it is possible to use a thin coating of conductive materialon an insulated lightweight base, rather than using solid aluminum, asin the prior art, is because advantage is taken of the electro-magneticphenomenon known as skin effect. It is known that at high freshowing onepreferred embodiment view of the embodiment illustrated packaging thegreater part of the electric current penetrates the conductor to only askin thickness with the depth of penetration depending inversely on thefrequency of the alternation of the electric waves. Theoreticalcalculations, as well as actual tests have demonstrated that foreffective use of Window against frequencies near 10,000 rnegacycles persecond the metallic coating required is of the order of only a fewmillionths of an inch. For frequency ranges of 100 to 500 megacycles persecond, a depth of from 20 to 40 millionths of an inch is required.

In the case of the ribbon type base of Figs. 1 and 2, the thickness ofthe base material which has been used successfully has beenapproximately .00045 inch with a width of approximately .036 inch. inthe case of a fibre type base, diameters on the order of .001 inch havebeen employed.

While the embodiment of Figs. 1 and 2 are most readily packaged in acontinuous spiral roll, two methods of packaging the fibre coatedembodiment of Figs. 3 and 4 have been illustrated in Figs. 5, 6 and 7.

Thus, in Fig. 5 is shown a method of packaging wherein the fibre coatedreflector cords 16 are arranged in a cylindrical container 17 with theaxis of the reflectors parallel to the axis of the container. With useof this method of packaging, it is seen that in a cylinder having adiameter of only 3 inches, an excess of 7 million coated fibershavingdiameters of .001 inch can be contained therein. Compared to prior artmethods of packaging solid window, there has been effected a vast savingof space.

A second method of packaging is illustrated in Figs. 6 and 7 wherein thecontinuous cord 16 is wound on a cylindrical core 18 to form acylindrical package. By

' cutting the package in half along a line parallel to the axis of thecore varying lengths of reflectors will be produced. Thus, assuming thecore has a diameter of one inch, and assume the full package is threeinches in diameter with a length of only two inches, approximately400,000 dipoles, varying in length from 1.57 inches to 4.6inches isproduced. Where the'frequency ofi operation of enemy radar is unknown,it is seen that the variation in length of the window produced is adesirable feature.

While certain specific embodiments of this invention have been describedherein, it is obvious that many modifications and adaptations willbecome apparent to those skilled in the art. It is the intent,therefore, that this invention shall cover all such modifications andadaptations, and that it be limited only by the prior art and the scopeof the appended claim.

'1 claim:

The method of producing radio Wave'reflector cords of varied lengthcomprising the steps of coating a continuous fibrous base with metal toform a reflecting surfaced cord, winding said cord about a core to forma package, and dividing the formed package into parts by cuttingtherethrough along a line parallel to the core axis.

